NASA has a clear need to develop new technology in support of its future mission objectives, whether they are beyond low earth orbit (BLEO) missions, the development of Lunar outposts, or the eventual exploration of Mars. As these missions develop, it is anticipated that crew members will spend extended time outside of space craft and established habitats, and therefore the agency is focused on the development of new, robust, lightweight life support systems for extra-vehicular activity (EVA). One area that is critical to life support systems is the control of CO2 and new space suits must be able to accommodate longer EVAs without increasing the size or weight of the primary life support system (PLSS). Since the lifetime of the sorbent currently used for CO2 control can be a limiting factor in EVA duration, the development of lighter, simpler, and reliable methods for CO2 control is a primary need to support advanced exploration. Indeed, previous works classify the development of advanced technologies for CO2 control as “critical” to NASA's current needs. (Barta, D. J. and M. K Ewert, “Development of Life Support System Technologies for Human Space Exploration”, SAE Paper 2009-01-2483, 39th Int. Conf. on Environmental Systems, Savannah Ga., 2009. Barta, D. J., M. K Ewert, M. S. Anderson, and J. McQuillan, “Life Support System Technology Development Supporting Human Space Exploration”, SAE Paper 2008-01-2185, 38th International Conference on Environmental Systems, San Francisco Calif. 2008.)
The rate of CO2 generation varies with the metabolic rate of the crew member. Recent studies of CO2 control technology have been carried out in which the CO2 injection rates were varied to match simulated metabolic rates. The average CO2 generation rate was determined to be 0.093 g/h per Btu/h of metabolic rate of activity (Wickham, D. T., Gleason, K. J., Cowley, S. C., Engel, J. R., and Chullen, C., “Advanced Supported Liquid Membranes for CO2 Control in EVA Applications”, SAE Paper 2013-01-3212, 43rd International Conference on Environmental Systems, Vail Colo., 2013). Assuming that the metabolic rate over an EVA is approximately 1000 Btu/h, then the average rate of CO2 production is 93 g/h. In addition, based on recent findings regarding the effect that CO2 has on decision making capability, NASA also has a current interest in reducing the maximum allowable CO2 concentration in the suit from 7.6 mm Hg to 2.8 mm Hg. (Wickham, D. T., Gleason, K. J., Cowley, S. C., Engel, J. R., and Chullen, C., “Advanced Supported Liquid Membranes for CO2 Control in EVA Applications”, SAE Paper 2013-3307, 43rd International Conference on Environmental Systems, Vail Colo., 2013.) Thus, in order to carry out EVA operations safely, the CO2 control system must be sized to handle at least average production rates for the duration of the EVA, which likely will last in excess of eight hours.
Current Methods for CO2 Control for EVA and on Space Craft
Currently, the Metox sorbent system, designed and constructed by Hamilton Sundstrand, is being used for CO2 control during EVA. The Metox employs a silver oxide sorbent, which reacts with CO2 at low temperature to produce silver carbonate. It is designed to maintain the CO2 pressure of less than 7.6 mm Hg at metabolic generation rate of up to 403 kcal/h. During an EVA operation, the silver is gradually converted to the metal carbonate and once all of the oxide has been converted, the canister is no longer effective. After the EVA, the canister is placed in a specially designed oven on board the spacecraft. It is then heated in a flow of air to about 200° C. causing the carbonate to decompose back into the oxide, regenerating the activity of the Metox for the next mission. Since the Metox canister cannot be regenerated during the EVA, its capacity can be a limiting factor in the mission duration and the only way to increase EVA time is to increase the size and weight of the canister.